command usermod not found
 

I want to modify my own primary group so that the second group becomes the first, but when I enter the command usermod -g secondgroup userid I get the message "command usermod not found"

Are you running this command as root? usermod lives in /usr/sbin, a directory reserved for special commands that is usually not included in regular users' PATH.

Hi and welcome to LQ!
As said by suicidaleggroll, probably you're running the command as normal user, instead of root.

just try -
$sudo usermod

Depends on whether sudo is enabled. Some distros do not enable it on user's accounts by default. If not, try this:
(enter root's password)
then your usermod command.